1. Field of the Invention
The present invention relates to an adjustment device for photographic lenses, particularly macrolenses of large focusing range.
2. Prior Art
In such lenses, two lens groups are displaced for the distance setting in order to assure good correction of the imaging errors of the lens over the entire focusing range. In this connection, a first lens group, which is generally identified as the front lens group, carries out a relatively large adjustment movement while a second lens group, the so-called floating group, carries out a smaller displacement.
It is common in known macro-objectives for the axial adjustment movement of both lens groups to be derived directly from the turning movement of the focusing ring. Thus it is known, for instance, from West German Pat. No. 22 27 448 and from West German Unexamined Application for Patent No. 23 39 723, to couple the front lens holder and the focusing ring which is in threaded engagement with the housing directly to each other and to drive a linearly-guided floating group via a second thread or a helical groove.
In this way, however,only relatively small adjustment movements of the front lens group can be affected. Furthermore, a front portion of the lens which is turnable upon focusing excludes the use of certain filter attachments and, thus, limits the possibilities of use for the lens.
Some macrolenses such as those which are, for instance, described in U.S. Pat. No. 3,914,026 therefore have a linearly guided front lens group which is coupled, via a first thread, with the focusing ring which in its turn is in engagement with the housing by means of a second thread. Upon turning of the focusing ring, the axial movement of the front lens group corresponds to the total of the pitches of the two threads. The floating group is also linearly guided and is driven by the focusing ring in the same manner.
In such lenses, however, the rotation of the focusing ring must be limited to an amount which is definitely less than 360.degree. in order not to endanger the mechanical stability of the lens. This limitation in the adjustment motion has the result that the macrolens described in U.S. Pat. No. 3,914,026 can be adjusted continuously only down to an imaging scale of 1:2 and that an additional intermediate ring with an expensive mechanism of transmission to the floating group is required for the range between 1:2 and 1.